Piezoelectric relay devices are recognized to provide a means for either initiating or interrupting current flow to a load device. A known piezoceramic relay device for this purpose is disclosed in U.S. Pat. Nos. 4,670,682 and 4,689,517, both assigned to the assignee of the present invention. The relay device includes a piezoceramic bender member formed by at least two planar prepoled piezoceramic plate elements secured in opposed parallel relationship sandwich fashion on opposite sides of at least one central conductive surface and having outer conductive surfaces that are insulated from each other and the central conductive surface by the respective intervening piezoceramic plate element thicknesses. Movable contacts associated with the movable bender coact with fixed contacts disposed thereby to either complete or interrupt an electrical circuit providing current flow from a power source to the load device. A representative form of this type relay device as disclosed in the above mentioned prior art patents employs a piezoceramic bender member which is selectively prepoled with clamping means secured at non-poled portions adjacent to and mechanically supporting the selectively prepoled bender member in a cantilever manner for operating pairs of coacting electrical contact means and with the non-poled portions being mechanically unstrained and electrically neutral. The bender member is made to operate either side of a center position normally assumed by the bender member in an unenergized position to thereby enable different modes of operation. In one mode of operation, the relay device can simply serve as an on-off switch wherein one pair of coacting switch contacts either makes or breaks the electrical circuit with respect to the load device. In a different mode of operation, however, the pair of coacting switch contacts is provided on each side of the bender member to enable selective energization of multiple load devices. Both modes of operation with the prior art "bimorph" type bender switching devices are further said to be conducted in a similar manner wherein the DC energization potential used to actuate deflection of the bender member has the same polarity as the polarity of the prepoling potential used to prepolarize the prepoled piezoceramic plate elements. The depolarization avoided by operating the relay devices in this manner provides dipole enhancement enabling relatively long term operation with load devices employing load voltages as high as 5000 volts and corresponding currents as high as hundreds of amperes.
In both above defined modes of operation, such piezoceramic relay devices have been recognized to afford major operational and structural advantages over either electromagnetic (EM) relays or semiconductor devices when employed in power switching applications. These advantages are reported in U.S. Pat. No. 4,658,154, also assigned to the present assignee, which further includes disclosure of piezoceramic relay switching circuits providing control of single and double load apparatus. The EM relays still widely employed for this purpose provide an interface between, for example, an electronic control circuit and a load circuit wherein the former handles the low power control signals for selectively energizing the relay coil to appropriately position the relay contacts coacting in the power circuit to switch relatively higher levels of power. When such relay contacts are closed, load current is conveyed, with virtually no losses, and when they are parted, load current is interrupted with the certainty only an air gap can provide. Over the years improvements in EM relays have resulted in increased efficiency and reduced physical size. That is, such relays can be actuated with control signals of rather low energy content to switch reasonably high levels of load current. For example, EM relays are available which can be actuated with a one watt control signal to switch several kilowatts of power at 115 or 230 volts AC. As a consequence, EM relays can be operated with signals generated by solid state control circuitry. On the other hand, the drawbacks associated with EM relays employed for controlling current flow in load circuits responsive to control signals still remain substantial. While current EM relays have been miniaturized as compared to earlier designs of the relays, their actuating power requirements are still quite large in contrast to, for example, state of the art solid state power switches. The current EM relays are still relatively complex and expensive to manufacture, for example, their coils typically require a multitude of turns of very fine wire. The coil resistance consumes some power which must be provided by a reasonably stiff power supply. When, for example, EM relays are utilized in home appliance controls, relay operating power must be derived from a 115 or 230 volt AC utility source. The requisite power supply, particularly when an EM relay is operatively associated with a solid state control circuit, requires a transformer, electrolytic capacitors, regulators and protection to insure a reliable source of relay actuating current. Such power supplies are both costly and constitute a significant source of power dissipation. Moreover, in certain applications where high ambient magnetic fields are present, such as in motor starter applications, EM relays must be specially shielded to discourage spurious operation. The drawbacks associated with employment of EM relays in power switching circuitry has thereby resulted in a trend toward utilizing solid state switches, such as SCRs, Triacs, Thyristors, MOSFETs, IGTs and the like as the power switching output device. While such solid state switches are becoming relatively inexpensive and may be smaller in physical size than comparably rated EM relays, they do present a rather significant "on" resistance, which, at high current levels; results in considerable power dissipation. Thus, semiconductor power switches being utilized in high current applications must be properly heat-sinked for protection against thermally induced damage, and, as a consequence, with their heat-sinks can take up more physical space than do their EM relay counterparts. Moreover, solid state power switches must be protected against possible damage in spurious operation as a result of transients, electrostatic discharges (ESD) and electromagnetic interference (EMI). All of these protective measures represent an additional expense. In that such solid state power switches do not impose an air gap to restrain the flow of current in their "off" condition and because of their "on" condition failure mode, Underwriters Laboratory has disapproved their application in numerous domestic appliances. Such disapproval has only been overcome in part with a combination of the solid state switches and series connected EM relays in some domestic appliances so as to provide the required air gap.
All of the foregoing major disadvantages found with employment of either EM relays or semiconductor switches as the power switching output device has prompted renewed interest in piezoelectric relays, including piezoceramic relay devices. Recent improvements in piezoceramic materials have enhanced their electromechanical efficiency for these relay applications. Piezoceramic drive elements may be fabricated from a number of different polycrystalline ceramic materials such as barium titanate, lead zirconate titanate, lead metaniobate and the like which are precast and fired into a desired shape such as rectangular-shaped ceramic plates. The piezoceramic relay devices require very low actuating current, dissipate minimal power to maintain an actuated state, and draw no current while in their quiescent or unenergized state. The electrical characteristics of the piezoceramic drive elements are basically capacitive in nature, and thus are essentially immune to ambient electromagentic fields. Such piezoceramic relay devices can be designed in smaller physical sizes than comparably rated EM relays. Since piezoceramic relay devices utilize switch contacts, contact separation introduces the air gap in the load circuit as required for UL approval in domestic appliance applications. Closure of these relay contacts provides a current path of negligible resistance, and thus unlike solid state power switches, introduces essentially no loss in the load circuit. Since additional structural and operational advantages for such improved piezoceramic relay devices can be found in the aforementioned prior art U.S. Pat. Nos. 4,670,682 and 4,689,517, both disclosures are hereby specifically incorporated into the present application in their entirety.
The suitability of such piezoceramic relay devices in controlling current flow to a particular apparatus understandably requires still other factors to be considered. Both the operational characteristics desired in the apparatus as well as the environmental conditions being encountered have to be satisfied. For example, one major domestic air-cooling apparatus, namely the household refrigerator employs refrigeration means to keep the contents of a storage compartment from spoiling while the defrost means are usually included to remove frost accumulation from the refrigeration evaporator surfaces. It is desirable to control the operation of this domestic appliance and still other similar air-cooling apparatus so as to operate directly from the available line voltage power supply with a minimal number and size of components in the control circuitry and while further reducing any susceptibility of the control circuitry to EMI and line transients. Employment of at least one piezoceramic relay device in the control circuitry could theoretically enable current flow to be switched between the refrigeration and defrost mechanisms in such air-cooling apparatus most efficiently with relative immunity to ambient electromagnetic fields. The piezoceramic relay devices are further particularly suited for use in combination with low powerdrain electronic circuit components to provide the control signals for actuation of the piezoceramic bender member and thereby enable the relay contacts to be either opened or closed. It would be further desirable to replace the separate control means now being employed in the representative air-cooling apparatus to individually control the refrigeration and defrost mechanism with a single control unit employing at least one piezoceramic relay device. Such simplification would prove particularly attractive for the above illustrated household refrigeration appliance since it permits more space to be utilized for food storage and makes it easier to contain the entire control means within the apparatus for a cleaner design appearance.
Understandably, the ability of a piezoceramic relay device and its associated control circuitry to function properly in a relatively high humidity and low temperature environment that is associated with atmospheric cooling apparatus represents a still further important consideration. The relay contacts must open and close reliably in this operating environment over the relatively long lifetime demanded for most refrigeration apparatus. While piezoceramic devices have been found capable of long term reliable operation, significant contact problems are recognized to still exist and which have heretofore only been ameliorated with additional circuit means being employed. Specifically, contact arcing is experienced for different reasons as the relay contacts are opened and closed and which has required additional circuit means to reduce wear and tear at the contact interface. The arcing problem occurring when the contacts are opened is attributed to a rise of reapplied forward potential across the contacts as they open which can be lessened with snubber circuits as proposed in both aforementioned U.S. Pat. Nos. 4,658,154 and 4,670,682. The arcing problem which occurs when these contacts are closed is attributed to mechanical contact bounce upon closure and this problem is dealt with in a still further commonly assigned U.S. Pat. No. 4,626,698. As therein proposed, novel circuits are utilized with a piezoceramic relay device including circuit means to initially impress a relatively low voltage energizing potential across the piezoceramic bender member to slow its movement and curtail contact bounce after initial contact closure. It is also proposed therein that such circuits be operated to extinguish current flow through the contacts when being opened to help alleviate the former arcing problem. The seriousness of both arcing problems can be appreciated from a still further recommendation appearing in the reference for utilization of specialized contact metals to withstand arc formation whenever the relay contacts are being separated.
Recent legislation in many states now requires domestic appliances to meet minimum energy efficiency standards. For a domestic air-cooling apparatus, which includes refrigerators, air conditioners, heat pumps and the like, such a requirement understandably dictates efficient use of electrical power in both refrigeration and defrost mechanisms. A major problem in this regard occurs from the manner in which defrost mechanisms are operated in a great many of the present day domestic air-cooling appliances. For example, a typical household refrigerator controls the compressor motor operation in the refrigeration mechanism by thermostat means as a function of the sensed temperature. The associated defrost mechanism is controlled by energizing a defrost heater operated with a timer at periodic intervals. Since the defrost time cycle is fixed by the manufacturer at the time of manufacture, no provision is made for varying the defrost cycle with changes occurring in the operating environment. Accordingly, the interval during which the defrost mechanism is permitted to operate remains constant despite wide fluctuations being encountered in the humidity environment when the apparatus is being operated by a user. It becomes possible thereby for the defrost mechanism to overheat the evaporator surfaces in a low humidity environment and which has occasioned the incorporation of additional thermostat means for precautionary interruption of the defrost cycle. Moreover, a current trend whereby the entire control system for a household refrigerator and still other domestic air-cooling apparatus is being housed within the particular apparatus for appearance sake and reduced wiring costs has further drawbacks. The defrost timer motor now becomes subject to very high ambient humidities and low temperatures while further contributing heat which must be removed by the refrigeration mechanism. The latter drawback understandably decreases the overall thermal efficiency of the particular apparatus thereby making it that much more difficult to comply with the aforementioned minimum standards upon energy efficency in domestic appliances. Upon considering these energy efficiency standards, it also becomes evident that a proper control of an air-cooling apparatus having both refrigeration and defrost mechanisms should further preclude any simultaneous operation of the respestive mechanisms.
As recognized in the aforementioned U.S. Pat. No. 4,658,154, the operation of a piezoceramic relay to regulate power input to a pair of resistive load devices in a manner precluding simultaneous operation of the respective devices can be carried out with minimum power consumption. Such operational control of the relay device as therein recognized employs high voltage integrated circuitry being powered directly from a conventional utility source such as available 115 volt or 230 volt AC power sources. For such relay control circuitry to efficiently and reliably regulate power input to an atmospheric cooling apparatus having either a refrigeration mechanism alone or having a refrigeration mechanism operatively associated with a defrost mechanism, however, requires that a number and variety of still other important criteria be met. As important criteria applicable to regulate power input to either mechanism, there is understandable need for efficient power consumption, long term reliable operation of the selected power switching devices, and relatively low costs associated with structural implementation of selected power switching means in a particular apparatus. Such criteria can further be illustrated in connection with control means now being exercised in a typical household refrigerator. The conventional refrigeration mechanism controls temperature in one or more food storage compartments with a thermostat that controls the compressor motor as a function of sensed temperature. The conventional defrost mechanism controls operation of the defrost heater with a timer motor so that a defrost cycle is initiated after a fixed operating time interval of the refrigeration mechanism has elapsed. Since the defrost timer cycle is generally fixed to provide suitable defrosting in a heavy-use high-humidity environment, there exists excess defrost capability when the apparatus is operating in a low humidity environment. It follows that energy utilization is often less than optimum in the conventional power control system and that a control algorithm wherein less energy is used would prove beneficial. Replacing the separate electrical control means which now operate the refrigeration and defrost mechanisms in the conventional apparatus with a single control unit employing a piezoceramic relay device has further benefits. Wiring costs in the apparatus can be reduced while reliability of operation would be increased due to simplier electrical interconnections. The advent of reliable and inexpensive microprocessor control circuits as evidenced by their wide use in many current domestic appliances makes it further possible to automatically provide the actuating signals to a piezoceramic relay device satisfying the above defined control algorithm for a typical air-cooling apparatus. In so doing, a commercially available integrated circuit chip device could be programmed with the necessary logic and timing commands to carry out a selected control algorithm and with the control signals derived in a manner serving to operate the further integrated circuitry which actuates the piezoceramic relay device. The resulting control circuitry could then be directly and ohmically connected to a suitable power source and the terminal means of the piezoceramic relay device for minimal actuating power requirements and thereafter respond automatically to deflect in a first direction to complete an electrical circuit between the power source and the refrigeration mechanism or to deflect in a second direction to complete an electrical circuit between the power source and the defrost mechanism, all in a relatively fail-safe manner.
Deriving more energy efficient as well as simpler and more reliable control means for electrical power regulation to either refrigeration or defrost mechanisms being employed in a typical atmospheric cooling apparatus requires a still more detailed understanding of the conventional control algorithm. In the above illustrated typical household refrigerator, the current control algorithm provides a daily fixed time interval for defrost rather than providing defrost as needed. To further explain, there are two electrical power control elements in the present refrigeration control system. The first is the conventional cold control which is a hydraulic bulb thermostat that controls the compressor motor as a function of sensed temperature. The conventional cold control algorithm, as distinct from a true temperature feedback control, exercises temperature control over the interconnected frozen and fresh food compartments in the refrigerator with a single temperature monitoring control point. Since the conventional refrigeration systems do not reach their peak efficiency unless the compressor is allowed to operate for a sufficiently long time period, such as from fifteen to forty-five minutes to reach steady-state conditions, the present cold control is an engineering compromise between a tight control of the temperature at the monitoring point and suitable operation of the refrigeration system itself. During this minimum compressor run time, the temperature in either one of the two compartments may overshoot the desired temperature, while attempting to minimize the overshoot results in lowering the energy efficiency of the refrigerator. It is further evident that whenever a warm thermal load is added to either of the food compartments that a thermal transient will be experienced. Adequate cold control is achieved by monitoring the temperature in the fresh food compartment. In the conventional refrigeration mechanism the temperature of the evaporator exit air is monitored using a long time constant thermostat such that short perturbations in the exit temperature will not cause the temperature control to actuate. The exit air from the evaporator section is cold compared to that from the fresh food compartment temperature, since it is a blend of the fresh and frozen food air flows. Therefore, the present cold control requires a significant hysteresis such as 10.degree.-13.degree. F. for stable operation. This hysteresis also guarantees the minimum run and off times for the compressor due to the time constant of the evaporator and thermostat. The minimum run time is required for efficient operation while the minimum off time is required for the refrigerant gas pressure to equalize so that the compressor motor will not attempt to start with a high back pressure. By utilizing the exit air temperature from the evaporator, a measure of both fresh and frozen food compartments is factored into the measurement since air from both compartments is mixed within the evaporator. The use of exit air temperature further permits a degree of thermal anticipation for the temperature control system, whereby the levels of temperature overshoot experienced in the fresh food compartments are minimized, thereby limiting damage to low thermal mass loads such as lettuce. User operated means are further provided in the conventional refrigeration mechanism to adjust the compartment temperatures in an otherwise closed loop temperature control arrangement. Calibration of the conventional cold control means is further dependent upon the altitude where the apparatus is being operated. In contrast to the temperature feedback control means for operation of the conventional refrigeration mechanism, the second defrost control means in the conventional apparatus initiates a defrost cycle with a timer controlling the defrost heater to operate over a fixed time interval. Typically, a 500 watt heater is provided to periodically melt the accumulated frost from the evaporator surface to maintain the energy efficiency of the refrigeration apparatus. Present defrost timers typically initiate a defrost cycle whenever the compressor in the refrigeration mechanism has operated for a fixed number of hours. This is accomplished by wiring the defrost timer in parallel with the compressor motor. The present defrost timer cycle is further customized for particular refrigerator designs wherein the defrost cycle is now initiated once for each eight to sixteen hours of compressor run time, depending on the particular refrigerator heat exchanger design. The conventional defrost heater is thereby energized for a maximum time of one-half hour followed by a short off time for the gas pressure and temperature in the refrigeration mechanism to equalize. The present fixed time defrost mechanism becomes inefficient when the refrigerator is operating in the low humidity environment or when operating at low usage wherein a very small amount of frost accumulates on the evaporator surfaces and results in the evaporator being taken to very high temperatures with the 500 watt heater being energized for a thirty minute time period required only for a heavy frost accumulation. By reason of such possibility for overheating, the conventional control system further generally includes a defrost termination thermostat system whereby the defrost heater power is interrupted whenever the evaporator reaches a preset temperature. Understandably, further incorporation of this energy inefficient control system within the food compartment of the apparatus not only subjects all components to a high humidity and low temperature environment but requires that additional thermal loads being generated by the control system itself also be removed by the refrigeration mechanism. That a more energy efficient control algorithm is needed seems further evident from the considerable activity now taking place to develop frost sensing means as a replacement for the conventional fixed time defrost mechanism.
It is a principal object of the present invention to provide a more energy efficient system for the regulation of electrical power to the refrigeration and defrost mechanisms employed in an atmospheric cooling apparatus.
It is still another important object of the present invention to provide control means employing at least one piezoceramic relay device to regulate electrical power input to an atmospheric cooling apparatus in a more fail-safe manner.
A still further important object of the present invention is to provide control means for the regulation of electrical power to an atmospheric cooling apparatus which does not employ electromechanical timer means.
Still another important object of the present invention is to provide improved electronic control means for automatic regulation of electrical power to an atmospheric cooling apparatus.
A still further important object of the present invention is to provide a novel method for regulation of electrical power to an atmospheric cooling apparatus.
Still a further important object of the present invention is to provide a method of operating a piezoceramic relay means to more efficiently regulate electrical power input to an atmospheric cooling apparatus.
Still another important object of the present invention is to provide a more efficient method automatically regulating electrical power input to an atmospheric cooling apparatus.
Another important object of the present invention is to provide a more efficient atmospheric cooling apparatus utilizing novel control means to regulate electrical power to the refrigeration and defrost mechanisms.
Still another important object of the present invention is to provide an atmospheric cooling apparatus employing simpler control means to regulate electrical power input with piezoceramic relay means.
A still further immportant object of the present invention is to provide an atmospheric cooling apparatus employing improved electronic control means to automatically regulate electrical power input to the refrigeration and defrost mechanisms.
A still further important object of the present invention is to provide a domestic atmospheric cooling apparatus utilizing novel control means to regulate electrical power to the refrigeration and defrost mechanisms.
Another important object of the present invention is to provide a household refrigerator employing novel control means to automatically regulate electrical power input between the refrigeration and defrost mechanisms.
These and still further objects of the present invention will become apparent upon considering the following detailed description for the present invention.